Trisomy 21, so called syndrome of Down, is a congenital chromosomal state caused by the presence of an additional chromosome to the even 21e.

The people having trisomy 21 show distinct clinical signs, a cognitive delay and morphological and physiological characteristics particular. However, these elements are variable from one person to another.

The Syndrome of Down owes its name to the English doctor John Langdon Down who published in 1866 the first description of the people reached of trisomy 21. Before him, other French doctors had made the observation of it.

As the people reached present features characteristic of the Mongols, i.e. a small head, a face round and flattened attached and drawn aside eyes, one named familiarly this disease “idiocy mongol” or the mongolism. Today, this denomination is rather pejorative.

In 1958, the French doctor Jérôme Lejeune identifies the cause of the syndrome of Down to knowing, an additional chromosome about the 21st pair of chromosomes. For the first time, the link is established between a backwardness and a chromosomal anomaly.

This discovery opened new possibilities for the comprehension and the treatment of many mental illness of genetic cause.

Research made it possible to determine that this chromosomal anomaly, which is the result of a nondisjunction, can arise in three different forms which cause three types of trisomy 21, that is to say free trisomy, mosaic trisomy and trisomy by translocation.

Free trisomy Free trisomy touches approximately 95% of the people having a trisomy 21. It is the result of an error of distribution of the chromosomes, which occurs during the first cellular division. It affects all the cells of the human body.

Mosaic trisomy Mosaic trisomy occurs during the second cellular division and touches approximately 2% of the population of the people having a trisomy 21. In this case, there is combination of cells which have 46 chromosomes and cells which have 47 of them.

The people having a mosaic trisomy compared present sometimes differences to the other people having a free trisomy: apparent physical features, cognitive potential more present, etc It is necessary however to avoid concluding that these people will be affected by trisomy, because one cannot know which cells comprise a third chromosome.

Trisomy by translocation In this type, one counts approximately 3% people having trisomy 21. Trisomy by translocation means that there was breaking of part of chromosome 21.

In this case, the child receives this chromosome transferred in his genetic luggage of the one of the parents, who is itself carrying, although it is not affected by the syndrome.

A meeting with a geneticist will make it possible to check the risks to give birth to another child carrying a trisomy.


Each human cell contains 46 organized chromosomes of 23 pairs on which the embarrassments are. At the time of the fecundation of an ovule and a spermatozoid , each relative transmits to his child 23 chromosomes, in other words, half of his genetic luggage.

Trisomy 21 is caused by the presence of a third chromosome 21, caused by an anomaly during the cellular division.

Chromosome 21 is smallest of the chromosomes: it counts approximately 300 genes. In 95% of the cases of trisomy 21, this surplus chromosome is in all the cells of the organization of the affected people. Rarer forms of the syndrome of Down are caused by other anomalies of the cellular division.

At approximately 2% of the trisomy people, one finds surplus chromosomes in only part of the cells of the organization. One calls that trisomy 21 mosaic.

At approximately 3% of the trisomy people, a part only of chromosome 21 is in excess. It is trisomy 21 by translocation.

The arrival in the family of a baby reached of trisomy 21 can require one period of adjustment. These children require particular care and an attention additional. Take the time to know your child and to make him room in the family.

Each trisomy child has his own personality and requires for as much love and support that the others. these children who reached trisomy 21 suffer sometimes from certain particular complications: Cardiac malformations. According to the Canadian Company of the syndrome of Down (SCSD), more than 40% of the children reached of the syndrome have a congenital cardiac malformation present as of the birth.

An occlusion (or blocking) intestinal requiring a surgery. It relates to approximately 10% of the new-born babies reached of trisomy 21.Une hearing impairment. A susceptibility to the infections such as for example pneumonia, because of a reduction in immunity.

An increasing risk of hypothyroidism (weak thyroid hormone), of leukaemia or convulsions. A delay of language, sometimes worsened by the hearing impairment. Disorders of the eyes and sight (the cataracts, strabismus , myopia or the farsightedness are more reply ). An increasing risk of apnea of the sleep. A trend with obesity.

At the men reached, a sterility. The pregnancy is however possible at most women. The adults reached are also more prone to the early form of the Alzheimer’s disease.

New treatments to come for trisomy 21 Two research orientations are targeted by the researchers who work to find a treatment to improve intellectual deficiencies due to trisomy 21, firstly while acting on genes responsible for the cognitive disorders and secondly while acting directly in the nervous system, in particular on the neurotransmitters (substances which relay information with the brain like acetylcholine, serotonin, the dopamine, the GABA, etc).

Several international research teams are implied in this research. The trisomy people count a chromosome 21 additional, which means approximately 300 genes in excess. Among those, only some are implied in intellectual deficiency. During last years, two genes were identified: the gene of the DYRK1A and the gene of CBS (cystathionine beta-synthase).

Clinical trials are in hand to discover molecules which would be able to inhibit the expression of these genes and thus to act on the degree of intellectual deficiency of the trisomy people. At the trisomy people, a neurotransmitters , the GABA (acid gamma-aminobutyric) is too abundant and causes a deceleration of the nervous activity.

Since 2011, researchers test the effectiveness of a molecule which could inhibit its action and improve the cognitive and behavioral deficits of the trisomic people.